The present invention relates to hearing aids and, more particularly, to earmolds that convey amplified sound from the hearing aid to the ear.
Audiologists have long sought to provide an earmold for a hearing aid that prevents the amplified sound from feeding back and interfering with the operation of the hearing aid and, simultaneously, to provide an earmold that is comfortable to wear. The hearing aid art is replete with devices that are able to meet one, but not both, of these objectives.
Feedback is the distortion of amplified sound caused by conduction of the amplified sound back to the microphone that receives the unamplified sound. Conduction occurs through the air pathway between the microphone and receiver in the hearing aid (acoustic feedback), and through the contact between the receiver and the surrounding housing (mechanical feedback). For hearing aid users with a profound hearing loss at several or all frequencies, the acoustic feedback problem is exacerbated by the need to generate abnormally loud sounds in the ear canal. For users with a partial hearing loss (for example, loss of hearing at high frequencies), resolution of the acoustic feedback problem is complicated by the need to amplify sound at some frequencies and to leave other frequencies unamplified.
The parts of the ear's anatomy pertinent to this invention are shown in FIG. 1. The ear canal 10 extends from the ear aperture 20 to the tympanic membrane 30. While canal size and shape may vary from person to person, it is generally about 24 millimeters long and has an S-shape. In cross section it is an oval with the major axis in the vertical direction near the aperture 20 and in the horizontal direction near the tympanic membrane 30. The cross-sectional area of the canal decreases at the isthmus 40 approximately 18 millimeters from the aperture. The canal is formed from cartilage 12 and bone 16 and is lined with skin. The cartilaginous portion is nearest the aperture 20 and is about 8 millimeters long. The osseous portion, formed from the temporal bone 16, is about 16 millimeters long. The temporal bone 16 also contains the cavities of the middle and inner ear. The region outside the ear canal adjacent the aperture 20 forms a bowl known as the concha 50.
Both the ear's anatomy and an incomplete understanding of the hearing process contribute to the failure to produce a hearing aid for both profound and partial hearing loss that comfortably reduced acoustic feedback. It is known, however, that the bones in the skull play an important role in hearing. The ear receives sound waves through the mechanisms of air conduction and bone conduction. Sound waves in the air move through an air conduction pathway (the ear canal) to the tympanic membrane, where they are conveyed to the inner ear. Sound waves also are received by the temporal bone of the skull and conveyed directly to the inner ear. In the inner ear sounds from both sources are joined to produce the full frequency spectrum of hearing. It is believed that the process of hearing may also include the reception of the pressure of acoustic waves on various neural receptors in the body which are relayed to the brain for interpretation along with the inner ear's signals.
Even if the body's methods for receiving and interpreting the various sensory signals which produce hearing were completely understood, and they are not, the hearing process is further complicated by the fact that the major signal source, the inner ear, receives acoustic signals which are complex waveforms dependent upon the size, shape, porosity, et cetera of the ear canal and its surrounding tissue. Sounds received within the ear canal are reflected, refracted and, in part absorbed by the ear canal and its surrounding structure. The sound which arrives at the ear drum has been altered by the various wave reflections and refractions within the ear canal and the head. Thus, the normal open-ear hearing process includes complex and little understood phase relationships among sounds arriving from the air and bone conduction paths. The loss or distortion of one of these paths by artificial devices can disrupt the normal phase relationships of the arriving signals.
One approach to reducing acoustic feedback in hearing aids has focused on blocking the air-conduction pathway. An acoustic barrier is placed in the ear between the receiver of the hearing aid and the outlet for the amplified sound. In one approach, the barrier is held in place by exerting pressure against the osseous and cartilaginous portions of the ear canal. See, for example, U.S. Pat. No. 4,006,796 to Coehorst dated Feb. 8, 1977, and U.S. Pat. No. 4,520,236 to Gauthier dated May 28, 1985. This pressure can be uncomfortable to the wearer and often results in the receding of the osseous and cartilaginous portions of the canal away from the pressure, i.e., the canal becomes greater in diameter. Because the barrier conducts amplified sound to the temporal bone, the normal phase relationships among sounds arriving from the air and bone conduction paths can be disrupted.
Other approaches have eliminated the pressure on the wall of the osseous portion of the canal and sealed the ear canal at the aperture or in the cartilaginous portions of the canal to obtain the desired reduction in feedback along the canal. See, for example, U.S. Pat. No. 3,061,689 to McCarrell, et al., dated Oct. 30, 1962, U.S. Pat. No. 3,312,789 to Lewis, et al., dated Apr. 4, 1967, and U.S. Pat. No. 2,939,923 to Henderson dated Jun. 7, 1960. These devices, however, do not deal with other problems caused by sealing the ear canal. These problems, insertion loss and occlusion effect, cause the hearing aid to produce sounds which are both unnatural and uncomfortable for the wearer.
Insertion loss is the removal of a portion of sound from the ear canal. Occlusion effect is the increased transmission of sound by bone conduction when air conduction is impeded. For example, one's own voice sounds different when one talks with his ears blocked. (See also, pp. 204-206 of "Bone Conduction" by Juergen Tonndorf in Foundations of Modern Auditory Theory, edited by Jerry V. Tobias, Vol. 2, pg. 197, Academic Press, New York.)
For those hearing aid users with partial hearing, the means to seal the ear canal in the devices in the above-cited patents indiscriminately disrupt the phase relationships for all frequencies, even those to which the otherwise malfunctioning ear may be responsive.
The present invention recognizes that the complex phase relationships of air and bone conduction are not completely understood. It creates a nearly natural hearing environment by reducing the interference with these complex relationships. Rather than blocking the ear canal with a massive seal, it opens the canal; rather than exerting pressure on the wall of the canal, it reduces wall contact. It reduces both feedback and insertion loss, and all but eliminates occlusion effect.
The present invention creates a critically tuned resonant cavity in the ear canal next to the tympanic membrane. The cavity is bounded by the wall of the canal, by the tympanic membrane, and by a flexible seal positioned in the canal, preferably between the isthmus and the tympanic membrane. The unimpeded sound received at the ear aperture moves relatively unimpeded through the canal until it reaches the face of the flexible seal nearest the aperture. Amplified sound from the hearing aid is conveyed through the ear canal inside the conduction tube and is released from the tube inside the resonant cavity. The flexible seal (whose primary function is to reduce acoustic feedback through the air conduction pathway) retains many of the natural phase relationships by (1) leaving much of the canal exposed to unamplified sound, and (2) vibrating at the frequencies of the unamplified sound. Because much of the canal is exposed, hearing aid users with normal hearing at particular frequencies are able to hear nearly natural sounds at those frequencies. Amplified sounds at the frequencies at which hearing is impaired are enhanced by the action of the resonant cavity. The resonant cavity restores much of the natural fullness of the sound by being in harmony with the frequencies of the unamplified sound.
It is accordingly an object of the present invention to provide a novel earmold for a hearing aid which obviates may of the problems of the prior art and which retains a substantial part of the natural hearing process.
It is another object of the present invention to reduce hearing aid feedback by exposing much of the ear canal to unamplified sound.
It is yet another object of the present invention to increase hearing aid user comfort by reducing the pressure on the wall of the ear canal.
It is a further object of the present invention to improve hearing aid performance and comfort by retaining many of the natural phase relationships among the sound pathways.
It is still a further object of the present invention to create a resonant cavity next to the tympanic membrane for retaining many of the natural phase relationships of the amplified frequencies.
It is yet a further object of the present invention to provide a method for making an earmold for a hearing aid that reduces feedback and is comfortable to wear.
These and many other objects and advantages will be readily apparent to one skilled in the art to which this invention pertains from a perusal of the claims and the following detailed description of preferred embodiments when read in conjunction with the appended drawings.